The effect of casting solvent and water on gas permeability of poly(γ-methyl L-glutamate) membrane

Gas permeability of poly(γ-methyl L-glutamate) (PMLG) membranes prepared by using dichlorethane, trifluoroacetic acid, and formic acid as solvents was studied. The membranes prepared by casting dichloroethane and trifluoroacetic acid solutions of the polymer, designated as PMLG—DCE and PMLG—TFA, respectively, had α-helical structures according to infrared absorption spectra, while the membranes prepared by allowing the PMLG—TFA membranes to swell in formic acid, designated as PMLG—FA, had mainly a β-sheet structure. The diffusion coefficients of each gas studied for PMLG—DCE, PMLG—TFA, and PMLG—FA decreased in that order, and the apparent activation energies of diffusion increased in that order. The apparent heats of solution for Ne,O₂, N₂, and CO₂ were negative in PMLG—FA. These results were discussed in connection with the molecular conformations of PMLG in the membranes. It is suggested that the diffusion of gases in PMLG—DCE takes place in the side-chain regions between helices, while in PMLG—FA the diffusion occurs across the polymer main chain whose mobility is depressed by the intermolecular hydrogen bonds. The effect of water on oxygen permeability of PMLG—DCE was small; on the contrary that of PMLG—FA was very large. Furthermore, it is assumed that the random-coil conformation partly exists in PMLG—TFA.